1. Field of the Invention
The present invention relates to well logging. In particular, the present invention is an apparatus and method for imaging of subsurface formations using electrical methods.
2. Background of the Art
Birdwell (U.S. Pat. No. 3,365,658) teaches the use of a focused electrode for determination of the resistivity of subsurface formations. A survey current is emitted from a central survey electrode into adjacent earth formations. This survey current is focused into a relatively narrow beam of current outwardly from the borehole by use of a focusing current emitted from nearby focusing electrodes located adjacent the survey electrode and on either side thereof. Ajam et al (U.S. Pat. No. 4,122,387) discloses an apparatus wherein simultaneous logs may be made at different lateral distances through a formation from a borehole by guard electrode systems located on a sonde which is lowered into the borehole by a logging cable. A single oscillator controls the frequency of two formation currents flowing through the formation at the desired different lateral depths from the borehole. The armor of the logging cable acts as the current return for one of the guard electrode systems, and a cable electrode in a cable electrode assembly immediately above the logging sonde acts as the current return for the second guard electrode system. Two embodiments are also disclosed for measuring reference voltages between electrodes in the cable electrode assembly and the guard electrode systems.
Techniques for investigating the earth formation with arrays of measuring electrodes have been proposed. See, for example, the U.S. Pat. No. 2,930,969 to Baker, Canadian Pat. No. 685,727 to Mann et al., U.S. Pat. No. 4,468,623 to Gianzero, and U.S. Pat. No. 5,502,686 to Dory et al. The Baker patent proposed a plurality of electrodes, each of which was formed of buttons which are electrically joined by flexible wires with buttons and wires embedded in the surface of a collapsible tube. The Mann patent proposes an array of small electrode buttons either mounted on a tool or a pad and each of which introduces in sequence a separately measurable survey current for an electrical investigation of the earth formation. The electrode buttons are placed in a horizontal plane with circumferential spacings between electrodes and a device for sequentially exciting and measuring a survey current from the electrodes is described.
The Gianzero patent discloses tool mounted pads, each with a plurality of small measure electrodes from which individually measurable survey currents are injected toward the wall of the borehole. The measure electrodes are arranged in an array in which the measure electrodes are so placed at intervals along at least a circumferential direction (about the borehole axis) as to inject survey currents into the borehole wall segments which overlap with each other to a predetermined extent as the tool is moved along the borehole. The measure electrodes are made small to enable a detailed electrical investigation over a circumferentially contiguous segment of the borehole so as to obtain indications of the stratigraphy of the formation near the borehole wall as well as fractures and their orientations. In one technique, a spatially closed loop array of measure electrodes is provided around a central electrode with the array used to detect the spatial pattern of electrical energy injected by the central electrode. In another embodiment, a linear array of measure electrodes is provided to inject a flow of current into the formation over a circumferentially effectively contiguous segment of the borehole. Discrete portions of the flow of current are separably measurable so as to obtain a plurality of survey signals representative of the current density from the array and from which a detailed electrical picture of a circumferentially continuous segment of the borehole wall can be derived as the tool is moved along the borehole. In another form of an array of measure electrodes, they are arranged in a closed loop, such as a circle, to enable direct measurements of orientations of resistivity of anomalies
The Dory patent discloses the use of an acoustic sensor in combination with pad mounted electrodes, the use of the acoustic sensors making it possible to fill in the gaps in the image obtained by using pad mounted electrodes due to the fact that in large diameter boreholes, the pads will necessarily not provide a complete coverage of the borehole.
U.S. Pat. No. 6,714,014 to Evans et al., the contents of which are fully incorporated herein by reference, discloses a device suitable for resistivity imaging with water based mud (WBM) and oil based mud (OBM). The device disclosed in the Evans '014 patent is basically a wireline logging device. U.S. Pat. No. 6,600,321 to Evans discloses a modification of the Evans '321 patent that is adapted for use in measurement while drilling (MWD) applications. Both of the Evans' patents have pad mounted electrodes that are in contact with the earth formation.
The devices described above are galvanic devices in which current is injected into the formation. With the exception of the Evans patents, they only work when the borehole is filled with a conducting fluid. U.S. patent application Ser. No. 10/657,870 of Ritter et al., filed on Sep. 9, 2003 and the contents of which are fully incorporated herein by reference, teaches the use of galvanic, induction and propagation resistivity devices for borehole imaging in MWD applications. The resistivity sensor may be mounted on a pad, rib, or a stabilizer. Specifically disclosed therein are a shielded dipole antenna and a quadrupole antenna. In addition, the use of ground penetrating radar with an operating frequency of 500 MHz to 1 GHz is disclosed. Ritter may involve an arrangement for maintaining the antenna at a specified offset from the borehole wall using, for example, hard facing.
An important aspect of any downhole electromagnetic tool is the ability to reduce power requirements. The devices discussed above do not address this issue. One parameter for estimating antenna performance is its electrical quality Q. This is important for antennas operating in a tuned mode (resonant tank) or transmitting device. Optimizing antenna quality would result in minimization of unproductive losses and improving tool measurement accuracy. The quality factor is given by
                    Q        =                              2            ⁢                                                  ⁢            π            ⁢                                                  ⁢            fL                    r                                    (        1        )            where f is the frequency, L is the inductance of antenna and r presents its active losses (or resistance). The Q is also an indication of the signal to noise ratio that can be achieved using an antenna. Maximizing Q can be done by maximizing the inductance and minimizing the resistance. An excessively large inductance can be a serious limitation in practical design as the antenna becomes sensitive to unavoidable stray capacitances and imperfections in the electronics.
Intrinsic losses in RF antennas are generally caused by a non-uniform current density in a conductor cross section and by eddy currents induced in a conductor by the magnetic field produced by a neighboring conductor. An alternating current in a conductor tends to flow in the proximity of its surface. The depth of current penetration inside the conductor is inversely proportional to the tool operating frequency and the electrical conductivity of the conductor. When transmitter and multiple receiver antennas are in close proximity to each other this effect can become even more significant.
Another issue that has not been addressed, particularly with respect to MWD resistivity tools, is that of conforming the antennas to a curved surface. Antennas currently produced have a rigid assembly that has to be deformed to fit the curved surface of a downhole MWD tool. A need exists for a downhole resistivity tool that has a flexible antenna, not affected by the type of borehole fluid, insensitive to stray capacitances, and has a high resolution. The present invention satisfies this need.